MIMO technology relies on multiple antennas to simultaneously transmit multiple streams of data in wireless communication systems. For example, MIMO is incorporated into recent and evolving wireless broadband standards such as 4G LTE and LTE-A, and allows a base station to communicate with several mobile terminals. The most modern standard, LTE-A, allows for up to 8 antenna ports at the base station.
Massive MIMO is an emerging technology that scales up MIMO, typically by orders of magnitude. Massive MIMO is sometimes abbreviated MaMi and is also known as “Large-Scale Antenna Systems”, “Very Large MIMO”, “Hyper MIMO”, “Full-Dimension MIMO” and “ARGOS”. Massive MIMO is a multi-user MIMO technology proposed for use in base stations for wireless communication, where each base station is equipped with an array of M active antenna elements, so-called service antennas, and utilizes these to communicate with K (single- or plural-antenna) terminals, over the same time and frequency band. The number M of service antennas may be at least 10, but is typically much larger, such as at least 20, 100 or 250. By coherent processing of the signals over the array, so-called transmit precoding may be used in the downlink to spatially focus each signal at its desired terminal, and so-called receive combining may be used in the uplink to discriminate between signals sent from different terminals. The more antennas that are used at the base station, the finer the spatial focusing to the terminals can be achieved. The spatial focusing enables use of a lower RF transmission power at both the base station and the terminals, without compromising signal-to-noise ratio (SNR). Massive MIMO is e.g. described in the articles “Massive MIMO: Ten Myths and One Critical Question”, by Bjornson et al, published in IEEE Communications Magazine, vol. 54, no. 2, pp. 114-123, February 2016, and “Massive MIMO for next generation wireless systems”, by Larsson et al, published in IEEE Communications Magazine, vol. 52, no. 2, pp. 186-195, February 2014.
The precoding and combining performed by a Massive MIMO base station rely on estimated channel responses to and from each of the terminals, denoted channel state information (CSI), and improve the effective SNR by a factor, denoted array gain, which is proportional to the number M and the quality of the CSI. Generally, the CSI may be different at the base station (uplink CSI) and the respective terminal (downlink CSI). The base station may obtain the uplink CSI by receiving and analyzing uplink pilots transmitted by the respective terminal. If the communication system is designed for TDD (time-division-duplex) operation, in which the same frequency bands are used for uplink and downlink, the downlink CSI will be equal to or at least approximate the uplink CSI, a phenomenon denoted “channel reciprocity”. Thus, the base station may rely on this channel reciprocity to use the uplink CSI for downlink precoding purposes, optionally after applying calibration coefficients that e.g. account for non-reciprocity in the transceiver radio frequency (RF) chains at both ends of the link. If the communication system is designed for FDD (frequency-division-duplex) operation, in which different frequency bands are used for uplink and downlink, the downlink CSI may be determined by the respective terminal, e.g. based on training signals transmitted by the base station, and transmitted back to the base station.
In both TDD and FDD, the base station is configured to communicate with the terminals on one or more channels with a predefined bandwidth. Thus, the base station comprises a dedicated RF receiver for each antenna, where the RF receiver is configured to select a desired channel to be translated to digital baseband.
As understood from the foregoing, the desirable array gain is only achieved when the base station has determined the uplink CSI based on the uplink pilots transmitted by the respective terminal. Thus, the base station needs to be able to receive the uplink pilots that are transmitted by a terminal without the use of the array gain. This could be achieved by the terminal using an increased transmission power for the uplink pilots. However, this approach of generating the uplink pilots has the drawback of increasing the power consumption at the terminal and adding requirements to the terminal HW.
Although the foregoing discussion has focused on terminals, such as mobile handsets, it is equally applicable to other communication devices, which may be either mobile or stationary. One particular application environment for Massive MIMO base stations may be the Internet of Things (IoT), in which all types of devices are integrated with the Internet. Such IoT devices with wireless communication capability include various types of sensors and controllers. Many of these IoT devices are low-complexity devices, e.g. embedded devices with limited CPU, memory and power resources. It is realized that the above-mentioned approach of ensuring that uplink pilots reach the base station is generally unsuitable for IoT devices with limited power resources.